1. Field of the Invention
This invention relates to a steel wire used as a wire rope, spring, cord for tire or the like, preferably used for the reinforcement of rubber articles, particularly a steel wire having an excellent fatigue resistance and a method of producing the same and a pneumatic tire using the same.
2. Description of Related Art
In general, the steel wires used as a wire rope, spring, cord for tire or the like are required to have various properties. For example, the weight reduction of the tire is urgently needed for recent environmental problems, particularly for promoting low fuel consumption of an automobile, and hence it is required to increase the strength of the steel wire as a starting material for the constituent of the tire to decrease the amount of the steel wire to be used.
In order to increase the strength of the steel wire, there are adopted a method of increasing C content in the steel wire, a method of increasing Mn or Cr content in the steel wire and the like. For example, although a general-purpose steel rod comprising C: 0.79-0.86 mass %, Si: 0.15-0.35 mass % and Mn: 0.30-0.60 mass % such as SWRH82A has been used as a starting material in the conventional steel wire, the use of a steel wire rod comprising C: 0.8-0.9 mass %, Si: 0.1-1.5 mass % and Mn: 0.1-1.0 mass % or a steel rod comprising C: 0.9-1.1 mass %, Si: not more than 0.4 mass %, Mn: not more than 0.5 mass % and Cr: 0.1-0.3 mass % is proposed for the increase of the strength.
However, such a high-strength steel wire is poor in the ductility and causes a problem that the fatigue resistance becomes poor. That is, when products using such a steel wire, particularly tires used under a severe environment are subjected to repetitive bending deformation, particularly when bending input or the like is applied to the steel wire, crack is easily created on the surface of the steel wire to bring about the breakage of the steel wire and hence troubles such as so-called belt breakage, side cut and the like are caused to lower the durability of the tire.
And also, the steel wire rod highly alloyed by increasing the C content or the like is poor in the ductility, so that when such a steel wire rod is drawn to produce steel wire, wire breakage is frequently caused and the production itself becomes difficult.
For this end, the wire breakage has hitherto been avoided by a multi-stage wet drawing method. As shown in FIG. 1, the multi-stage wet drawing method using a die 4 at every stage is effective to avoid the wire breakage during the drawing in a process that a steel wire 3 is alternately extended between driving capstans at each stage in multi-stage driving capstans 2a, 2b disposed in a lubricating liquid 1.
However, steel wires obtained by such a multi-stage wet drawing method, particularly steel wires obtained by drawing high-carbon steel wire rod having a C content of not less than 0.6 mass % have a problem that when a plurality of such wires are used to produce a steel cord, the wire breakage is frequently caused at a step of twisting these wires.
It is, therefore, an object of the invention to provide a steel wire simultaneously establishing the increase of the strength and the improvement of fatigue resistance to realize the weight reduction and the improvement of the durability of products using such a steel wire, particularly rubber articles using such a steel wire as a reinforcing member.
It is another object of the invention to provide a drawing method useful for obtaining steel wires having high strength and ductility from high-strength steel wire rod represented by a high-carbon steel wire rod.
According to a first aspect of the invention, there is the provision of a steel wire having a diameter of not more than 0.40 mm, a tensile strength of not less than 4000 MPa and number of torsion of not less than 15.
In a preferable embodiment of the first aspect, the diameter is not more than 0.35 mm and the tensile strength is not less than 4300 MPa, more particularly 4500 MPa.
In another preferable embodiment of the first aspect, the number of torsion is not less than 20.
According to a second aspect of the invention, there is the provision of a method of producing a steel wire as defined in the first aspect of the invention by subjecting a steel wire rod to multi-stage wet drawing, which comprising conducting at least a final stage of the multi-stage wet drawing with a die having an inner peripheral wall made from a single crystal diamond chip or a sintered diamond chip having a diamond particle size of not less than 10 xcexcm.
In a preferable embodiment of the second aspect, a wire drawing quantity xcex5n represented by the following equation is not less than 4.0 at the final stage of the multi-stage wet drawing:
xcex5n=2xc2x7ln(d0/dn)
wherein
d0: diameter of wire rod before the multi-stage wet drawing;
dn: diameter of wire delivered from a die at n-th stage.
In another preferable embodiment of the second aspect, dies having an inner peripheral wall made from a single crystal diamond chip or a sintered diamond chip having a diamond particle size of not less than 10 xcexcm are used in such stages of the multi-stage wet drawing that a wire drawing quantity xcex5n represented by the following equation is not less than 4.0:
xcex5n=2xc2x7ln(d0/dn)
wherein
d0: diameter of wire rod before the multi-stage wet drawing;
dn: diameter of wire delivered from a die at n-th stage.
In the other preferable embodiment of the second aspect, dies having an inner peripheral wall made from a single crystal diamond chip or a sintered diamond chip having a diamond particle size of not less than 10 xcexcm are used as such dies that a wire drawn out from the die has a tensile strength of not less than 4000 MPa.
In a still further embodiment of the second aspect, the diamond particle size in the sintered diamond chip is not less than 15 xcexcm.
In a further preferable embodiment of the second aspect, the die has an approach angle of 6-12xc2x0, preferably 7-10xc2x0 and a bearing length corresponding to 30-50% of a diameter of a wire delivered from the die.
In the other preferable embodiment of the second aspect, the steel wire rod has a carbon content of not less than 0.60 mass %, preferably not less than 0.80 mass %.
According to a third aspect of the invention, there is the provision of a pneumatic tire comprising a carcass extending between a pair of bead portions and a belt comprising plural belt layers superimposed on the carcass, characterized in that the steel wire defined in the first aspect of the invention is used as a reinforcing member for the carcass and/or the belt, or a steel cord obtained by twisting a plurality of the steel wires defined in the first aspect of the invention is used as a reinforcing member for the carcass and/or the belt.